Retrieves embedded metadata timestamps from Kafka messages.
If a record has a negative (invalid) timestamp value, this extractor raises an exception.

Embedded metadata timestamp was introduced in "KIP-32: Add timestamps to Kafka message" for the new
0.10+ Kafka message format.

Here, "embedded metadata" refers to the fact that compatible Kafka producer clients automatically and
transparently embed such timestamps into message metadata they send to Kafka, which can then be retrieved
via this timestamp extractor.